Of course, the cost of Arena varies greatly based on how you play the game
The best way to make sure your modular creatures die is to do it yourself
The simplest version of an Iron Spider, Stark Upgrade infinite combo involves Crystalline Crawler and some sort of untap tool
For graveyard-heavy decks such as Reanimator, this lets you dump the cards you want into the graveyard there while putting in reanimation enablers into your hand
For reprints, we have Asceticism, Lightning Greaves, and Swan Song as decently valuable and powerful cards
Make sure to let me know if this type of stuff interests you, because I could definitely do an entire article on promos in general, and maybe even another one on promo lands